The present invention relates to a method for (1) inactivating enzymes, microorganisms and spores in a liquid foodstuff, and (2) sterilizing microorganisms, spores and virus in a liquid foodstuff. More particularly, the present invention relates to a method for (1) inactivating enzymes, microorganisms and spores in a liquid foodstuff, and (2) sterilizing microorganisms, spores and virus in a liquid foodstuff, which comprises contacting the liquid foodstuff with carbon dioxide in a supercritical state supplied through a filter, so that this method is highly safe in practice thereof and, further does not require specific high-temperature heating step or heating step under reduced pressure.
Soy sauce which is one of fermented foodstuffs, is a typical Japanese seasoning which has gained popularity overseas along with dishes of Japanese cuisine in recent years. Soy sauce, however, is disliked in part by Japanese and foreigners because of its peculiar smell. It is, therefore, considered a key factor for elevating the utility and value of soy sauce to remove the peculiar smell (odor) of soy sauce and to enhance its mild aroma.
Sake, which is another fermented foodstuff, is generally produced through the following steps. In the first step, fermented rice is compressed and filtered to obtain shinshu (green sake). In the second step, this obtained green sake is sterilized by heating at 60.degree. to 65.degree. C. and stored. In the third step, a number of lots of the stored sake which have been heat-treated are properly blended to determine the sake quality (sweet or dry, etc.) and the alcohol content is adjusted to standards. In the fourth (final) step, the thus prepared sake is again sterilized by heating and then bottled or packed. Thus, in the production of sake the heat-treatment is conducted twice, in the second and forth steps, in the manufacturing process, to effect inactivation of enzymes and sterilization.
The fresh flavor of green sake is sharply reduced by two heat-treatments in the manufacturing process, so that consumers request fresh sake which has been produced without undergoing any heat-treatment. However, green sake, although fresh in taste and smell, is very changeable in flavor because of its high enzymatic activity. There are marketed some brands of non-heat-treated sake sold at low temperature as fresh sake, but such fresh sake is subject to deterioration of quality by the action of certain enzymes such as .alpha.-amylase, protease and carboxy-peptidase, which poses the problem of increased distribution costs to prevent qualitative deterioration.
In order to solve the above problem, attempts have been made to remove such enzymes in the green sake by precision filtration or ultra-filtration, but sufficient removal of the enzymes has been impossible with such means. Further, the enzyme removal using a membrane has the problem that the aromatic components of green sake may be dissolved in the membrane during the process and/or that some kinds of taste components of green sake may be removed, thereby losing its relish.
Also, with reference to muddled fruit drinks such as orange juice, etc., it is considered that the cloud of the drink is a decisive factor for the quality of the drink. Inactivation of pectin esterase (PE) is essential for maintaining the stability of this cloud, but since PE is an enzyme stable to heat, usually a heat-treatment under a high-temperature condition (88.degree. to 99.degree. C. or 120.degree. C.) is required for inactivation of this enzyme. However, the heat-treatment at such high temperature causes deterioration of freshness and characterstic taste of the drink.
On the other hand, Japanese Patent Application Laid-Open (KOKAI) No. 3-27268 discloses a method for producing soy sauce, comprising bringing soy sauce into contact with carbon dioxide kept in liquid state or supercritical state to obtain soy sauce reduced in smell characteristic thereof and having a good disappearance of foam.
U.S. Pat. No. 4,714,591 discloses a method to recover valuable lignin and other extractable components from kraft black liquor and remove undesired components therefrom using supercritical fluids and convert the recovered lignin into chemical products. Further, U.S. Pat. No. 4,714,591 discloses a method for inactivating enzymes, comprising passing supercritical carbon dioxide through orange juice. JOURNAL OF FOOD SCINCE, Vol. 56, No. 3, 743-746, 1991 discloses a method for inactivating enzymes, comprising passing supercritical carbon dioxide through orange juice. U.S. Pat. No. 3,966,981 discloses a process for removing residual solvent from materials containing the same which involves extracting the material with liquid CO.sub.2, separating the phases, and evaporating CO.sub.2 from the treated material. Agric. Biol. Chem., 51 (2), 407-412, 1987 discloses a method for sterilizing microorganisms with supercritical carbon dioxide. Hakkokogaku, Vol. 67, No. 5, 439-457, 1989 disclosures a supercritical carbon dioxide extraction method.
However, there is no teaching nor suggestion of supplying carbon dioxide of a supercritical state as a micro-particle fluid state into a reactor by means of a filter in Japanese Patent Application Laid-Open (KOKAI) No. 3-27268, U.S. Pat. No. 4,714,591, JOURNAL OF FOOD SCINCE, Vol. 56, No. 3, 743-746, 1991, U.S. Pat. No. 3,966,981, Agric. Biol. Chem., 51 (2), 407-412, 1987 and Hakkokogaku, Vol. 67, No. 5, 439-457, 1989. By the methods disclosed in the above-mentioned publications, it is impossible to obtain sufficient amelioration of smell, inactivation of enzymes and sterilization. Further, it is impossible from the above-mentioned publications to expect the supplying carbon dioxide of a supercritical state as a micro-particle fluid state through a filter with a specific mesh size for (1) inactivating enzymes, microorganisms and spores in a liquid foodstuff, and (2) sterilizing microorganisms and virus in a liquid foodstuff.
As a result of intensive studies for solving the above problems, it has been found that by contacting a liquid foodstuff such as fermented liquid foodstuffs and various kinds of fruit juice with carbon dioxide of a supercritical state as a micro-particle fluid state, supplied through a filter with a mesh size of not more than 120 .mu.m in average diameter in a treating tank, the desired inactivation of enzymes, microorganisms and spores, and sterilization of microorganisms, spores and virus can be accomplished effectively. On the basis of the finding, the present invention has been attained.